The invention relates to a method and an apparatus for monitoring and controlling an appliance or a system comprising a number of appliances, having a two-dimensional or multidimensional operating envelope, whose limits represent different technical, procedure, financial or contractual restrictions.
One example of an appliance having a two-dimensional (planar) operating envelope, which is referred to as an operating characteristic, is a compressor. The gas volume flow V and the enthalpy difference H are chosen, for example, as characteristic variables for the operating point of a compressor. These characteristic variables are used as co-ordinate axes for representing the operating characteristic, which is defined as the totality of all those points (operating points) at which the compressor may be operated in the Vxe2x80x2H plane. In practice, the operating characteristic is mainly in the form of a curved polygon. The sides of the polygon are referred to as bounds whose infringement results in certain secondary conditions no longer being satisfied. One of the bounds is, for example, the pump bound, which defines a minimum gas volume flow at which there is still protection against pump surges; beyond the pump bound, operation of the compressor becomes unstable. Further bounds correspond to the maximum rating of the compressor drive, the maximum flow rate, and the minimum and the maximum rotation speed.
The monitoring of a compressor has two tasks:
1. Preventative monitoring, to protect the equipment against dangerous and unstable operating conditions, for example protection against pump surging.
2. Functional monitoring in order, for example, to set a specific compressor rotation speed as a function of the operating state (for example the load), or to maintain a predetermined nominal value.
Known monitoring systems for compressors are based on the measurement of a number of characteristic variables for the compressors and their controllers. If the operating point infringes what is referred to as a control line, which runs parallel to a bound, and at a specific distance from it, in the operating characteristic, suitable measures are taken in order to return the operating point to an area on this side of the control line once again.
In EP-B-0 332 888, the rate of movement of the operating point in the direction of the pump limit in the operating characteristic is determined; if a pump control line, which occurs at a variable distance from the pump limit depending on the speed, is infringed, a blow-out or bypass valve is opened quickly, in addition to the normal blow-out control.
U.S. Pat. No. 3,994,623 discloses the linking of a number of originally independent control loops. A method is proposed which can be carried out using a cascade circuit. The method comprises the control loops for the rotation speed, the feed pressure and the gas volume flow being linked to one another, with the output signal from each outer loop representing the input signal for the next inner loop.
In known control methods, a separate control loop is provided for each of the control lines. If a control line is infringed, measures are taken, which depend on the respectively infringed control line and its specific shape. These methods are thus based on an independent description of each individual control line. If the operating point is in an area close to two bounds (a xe2x80x9ccornerxe2x80x9d of the operating envelope), control of the operating point is very complicated. The control loops associated with the various bounds thus often operate independently of one another and conflict with one another, or else the interaction of the various control loops is incomplete.
Similar difficulties also occur when controlling other appliances or systems which comprise a number of appliances, having an operating envelope with two, three or more dimensions.
In the case of a three-dimensional operating envelope, the boundary conditions, which must not be infringed in either direction, are generally represented as, possibly curved, surfaces. The operating envelope is then an irregular polyhedron with curved side surfaces. If the operating point is in the vicinity of more than one of the side surfaces, this results in a complicated control response. In the general case of an N-dimensional operating envelope, the boundary conditions can generally be represented as, possibly curved, (Nxe2x88x921)-dimensional hyperplanes. In this case as well, particular control difficulties arise when the operating point approaches more than one of the hyperplanes.
A first object of the present invention is to provide a standard method for monitoring or controlling the position of the operating point in a two-dimensional or multidimensional operating envelope, which is independent of the specific shape of the operating envelope and which avoids said difficulties when the operating point approaches the limits of the operating envelope. The method is intended to allow the user to be warned reliably of unacceptable or dangerous operating conditions (monitoring), and to avoid them, or to maintain a predetermined nominal value (control).
This object is achieved by a method for monitoring or controlling an appliance or a system comprising a number of appliances, having a two-dimensional or multidimensional operating envelope, in which, according to the invention, the operating envelope, or a part of it, is mapped onto a unit domain or onto the operating envelope of another appliance for monitoring or control.
In this method, an operating point or set point in the operating envelope is advantageously transformed by means of the map to a point in the unit domain or in the operating envelope of the other appliance. The monitoring or control is in this case carried out on the basis of the position of the image of the operating point which results from the map.
The term unit domain in this case means intrinsically any desired region which is selected and is cohesive as an entity. For two-dimensional operating envelopes, for example, the unit circle or a half plane can advantageously be used as the unit domain. The unit sphere or a semi-infinite body advantageously carries out this role for operating envelopes with more dimensions.
If a map is made of the operating envelope onto a unit domain, the method according to the invention can be used to identify when the operating point is approaching a limit of the operating envelope, since the image of the operating point approaches a limit of the unit domain. One advantage of the method according to the invention is in this case that, provided the unit domain, the mapping rule and the co-ordinate representation of the image are selected appropriately, the situation where the image of the operating point is approaching a limit of the unit domain can be defined and controlled by monitoring a single limit.
A further advantage of the method according to the invention is that the monitoring of the operating point of different appliances can be carried out in a standard way by using differently shaped operating envelopes.
If the operating envelope of the appliance to be monitored is mapped onto the operating envelope of another appliance, the operating point of the appliance to be monitored can be monitored and controlled in the same way as for the other appliance. In particular, in this way, methods which have been proven for monitoring the operating point of a specific appliance can be transferred to the monitoring of the operating point of any other desired appliances with operating envelopes of any shape in the same dimension.
The method according to the invention is suitable for a large number of fields of application, in particular for monitoring a compressor.
In the method according to the invention, one or more process parameters are advantageously determined by means of the map, or the inverse transformation with respect to it.
In one advantageous refinement, the method for monitoring according to the invention contains the following steps:
a) determination of the operating point in the operating envelope;
b) transformation of the operating point by means of a stored map rule for the map;
c) outputting the co-ordinates of the image of the operating point.
In one advantageous refinement, a control method according to the invention contains the following steps:
a) determination of the operating point in the operating envelope;
b) transformation of the operating point by means of a stored map rule for the map;
c) comparison of the position of the image of the operating point with bounds, control lines or with the image of a set point in the image area;
d) determination of control parameters which define a control action;
e) carrying out the control action.
For a two-dimensional operating envelope, the method according to the invention can advantageously be implemented such that the map transforms a polygon, which is similar to the operating envelope, to a unit domain or an operating envelope of another appliance. The mapping can then advantageously be carried out by means of a function defined by the Christoffel-Schwarz integral. In this case, it is advantageous to approach the operating envelope from the inside. This means that the polygon which is similar to the operating envelope is located entirely in the interior of the operating envelope.
A further object of the invention is to provide an apparatus for carrying out the method for monitoring and control.
This object is achieved by an apparatus for monitoring or controlling an appliance or a system comprising a number of appliances having a two-dimensional or multidimensional operating envelope in which, according to the invention, means are provided for determining and storing parameters which are required to produce a map which maps the operating envelope, or a part of it, into a unit domain or into the operating envelope of another appliance.
The apparatus advantageously furthermore contains means for producing the map for any desired point in the operating envelope.
Furthermore, it is advantageous for the apparatus to contain means for comparing images of two points located in the operating envelope, with the comparison being carried out in the unit domain or in the operating envelope of the other appliance.
In a further advantageous refinement, the apparatus contains means for comparing an image of a point which is located in the operating envelope with at least one line (in particular a control line or bound) in the unit domain or in the operating envelope of the other appliance.
Furthermore, the apparatus may contain means for determining one or more process parameters by means of the map of the operating envelope onto the unit domain or onto the operating envelope of the other appliance.
An apparatus just for monitoring the operating point of an appliance or of a system comprising a number of appliances in this case advantageously contains the following parts:
a) means for determining the operating point of the appliance or of the system comprising a number of appliances;
b) means for producing a map of the operating point, which maps the operating envelope or a part of the operating envelope onto a unit domain or the operating envelope of another appliance;
c) means for outputting the co-ordinates of the image of the operating point.
An apparatus for controlling an appliance or a system comprising a number of appliances can advantageously contain the following parts:
a) means for determining the operating point of the appliance or of the system comprising a number of appliances;
b) means for producing a map of the operating point, which maps the operating envelope or a part of the operating envelope onto a unit domain or the operating envelope of another appliance;
c) means for determining parameters which define a control action;
d) means for carrying out the control action.
Finally, an advantageous refinement of an apparatus for control may contain the following parts:
a) a first computation model for transformation of a set point to a point in the unit domain or in the operating envelope of the other appliance;
b) a measuring unit for determining the position of the operating point;
c) a second computation module for transformation of the operating point to a point in the unit domain or of the operating envelope of the other appliance;
d) a comparison unit, which compares the transformed operating point with the transformed set point or with at least one bound or control line of the transformed operating envelope;
e) a monitoring unit for determining the parameters required for control,
f) and an execution unit for carrying out the operations required for control.
In this case, the first and second computation models may also be identical; in this case, the (single) computation module transforms both the set point and the operating point.
A method according to the invention for monitoring and controlling the position of the operating point, and the principles required to understand this method, will be explained in more detail in the following text.
From the mathematical point of view, an operating envelope is generally represented by a integral cohesive region. It is then always possible to produce a continuous map, which can be differentiated and is reciprocal, of the operating envelope onto the interior of a unit domain (that is to say of another integral cohesive region).
Furthermore, mapping of the operating envelope onto a unit domain, and subsequent mapping of the unit domain onto the operating envelope of another appliance, in principle allows any N-dimensional operating envelope to be mapped onto an N-dimensional operating envelope of another appliance.
Particularly in the case of two-dimensional operating envelope, the co-ordinate axes may be regarded as the real and imaginary axes in the complex number plane. Riemann""s mapping rule for complex analysis then ensures that a unique (reciprocal) and conformal map onto the interior of the unit circle exists for each integral cohesive region.
In order to carry out the monitoring and control method, a mapping rule for mapping the operating envelope or a part of it onto a unit domain or the operating envelope of another appliance is first of all stored in some suitable form.
The stored mapping rule may, for example, be a computation rule for calculating the image of each point within the operating envelope, or a part of it. In the simplest case, this computation rule is an analytical formula or an approximation formula for the map. Alternatively, the mapping rule may, for example, comprise a table which contains the associated image points for selected points in the operating envelope (original image point), possibly together with an interpolation rule on the procedure to be adopted for intermediate points in the operating envelope.
The stored mapping rule depends on the specific shape of the operating envelope, and is characteristic of the appliance to be controlled. Generally, the storage process is carried out only once for a specific appliance, provided the appliance characteristics do not change (for example due to ageing).
For the actual monitoring procedure, the operating point is first of all determined in the operating envelope. This step is expediently carried out by means of measuring appliances which are suitable for detecting the characteristic variables of the appliance to be controlled. The measured values are converted to co-ordinate values in the operating envelope. The operating point then consists of a set of N co-ordinate values, corresponding to the N dimensions of the operating envelope. In the next step, the operating point is mapped by means of the stored mapping rule. The image of the operating point is now in the form of a set of N values of transformed co-ordinates, which no longer need to correspond directly to the characteristic variables of the appliance to be monitored.
In the next step, comparisons are carried out on the basis of the position of the image of the operating point. To this end, for example, the position of the image of the operating point is determined with respect to the boundary of the image area. Alternatively, or additionally, the position of the image of the operating point can be compared with the position of the image of a set operating point, in which case the set operating point may be defined once or may be continuously matched to the operating requirements. It is likewise possible for the rate at which the image of the operating point is approaching the limit of the image area, or the image of the set operating point, to be determined as well.
Suitable control parameters are defined on the basis of such information. This may be done, for example, by first of all determining the direction, in the transformed co-ordinates, in which the image of the operating point is intended to be moved within the image area as a result of the control action. This direction may be, for example, the direction vector from the image of the operating point to the image of the set operating point, or a normal vector, pointing into the interior of the image area, on the boundary of the image area.
Now, firstly, the direction vector just determined can be transformed by means of the map which is in the inverse of the stored mapping rule to the co-ordinates of the original operating envelope, and a specific control action can be defined from the resultant original image of the direction vector. In this case, the inverse mapping process can be carried out, in a similar way to the original mapping process, by means of a computation rule or a table from original image and image points or from original image and image vectors. If the inverse mapping process is carried out by means of a table, then this table may be the same as that which was used for the original mapping of the operating point.
On the other hand, the determined direction vector in transformed co-ordinates can instead of this be used directly for defining the parameters for the specific control action. It is advantageous for this purpose for a table to have previously been defined in which the movement direction of the image of the operating point to be expected in the transformed co-ordinates has been entered for specific control actions, which are to be carried out once. Suitable control actions for recording in such a table are, in particular, those actions during which only a single operating parameter is changed. A specific control action, which may comprise the changing of a number of operating parameters, can then be defined by means of this table.
Finally, in the last step of the proposed method, the control action defined in this way is carried out on the basis of the determined control parameters.
Furthermore, an apparatus for monitoring the operating point according to the invention will now be described in more detail.
First of all, this apparatus contains means for determining the operating point. These means may be, for example, measuring appliances which, at their output, produce a signal which represents a unique measure of the value of a characteristic variable. The signals produced by the measuring appliances thus represent the operating point.
Furthermore, the apparatus contains means for producing the map of the operating point. These means may, for example, comprise an analogue/digital converter, a computation unit and a memory unit. The analogue/digital converters convert the signals which represent the operating point to a corresponding digital value. The operating point is now represented by N digital values.
A computation unit uses these values and a mapping rule which is stored in a memory unit to produce new digital values, which represent the co-ordinates of the image of the operating point in the unit domain. The mapping rule may in this case be located in the memory unit, for example in the form of a computation rule, or may be in the form of a table with an interpolation rule. Instead of being carried out digitally, the calculation can also be carried out by analogue means, without using analogue/digital converters. The means for producing the map of the operating point then contain an analogue computation device.
Finally, means are provided for outputting the co-ordinates of the operating point. These may be, for example, a screen for producing a graphic display of one or more co-ordinates of the image of the operating point, or digital/analogue converters with downstream display instruments.
An apparatus which is intended to be used not just for monitoring and displaying the operating point, but also for controlling it, contains further components.
Thus, furthermore, an apparatus such as this has means for determining the parameters which define a control action. These may comprise, for example, a computation unit, a memory unit and a digital/analogue converter. The computation unit uses the position of the image of the operating point in transformed co-ordinates to calculate the parameters which define a control action; in the process, it accesses computation rules stored in the memory unit, or tables for calculating these parameters. The digital/analogue converter converts these parameters into signal values. The means for carrying out the control action then use these signal values to directly influence the response of the appliance or of the system to be controlled.